Fix a bug with --detect-libgrades and --make.

compiler/mercury_compile.m:
	Ensure that any extra flags introduced by --detect-libgrades
	get put into mmc --make's option cache.  If this does not
	happen the correct set of library grades will not be passed
	to processes invoked by mmc --make.

	Add --verbose and --very-verbose output to library grade
	detection.